The present invention relates generally to packing material laminates and their manufacture. More specifically, the present invention relates to a laminate including a layer of a metal foil, in particular aluminium foil, together with one or more layers of plastic film. A method for the manufacture of such a laminate is also disclosed.
Aluminium foil has been used for a long time in connection with packaging as an item included in the laminate together with plastic material, or paper layers and plastic material. When a material for packaging of a certain product is chosen, the properties of the packing material must be adapted to the requirements of the product in respect of a protective barrier. A liquid product thus must be packed in a liquid-tight and liquid-resistant material, an oily product in an oil-resistant material and so on. In order to be able to seal the packages in an effective and convenient manner, the inside of the packing material is often provided with a heat-sealable plastic layer, and if stiffness of the packing material is required, a layer of paper or similar material is generally used. In other words a packing material is prepared, which has the properties which are desired, by laminating different material layers to each other. One characteristic that is desirable for many packing material combinations is tightness against gases, especially oxygen gas. Most plastic materials have poor oxygen gas barrier properties and it is for this reason, if gas-tightness is required, that in general a layer of metal foil, in particular aluminium foil, is incorporated in the packing laminate. An aluminium foil has outstanding gas-tightness characteristics even if the foil is extremely thin (e.g. 5 .mu.m) and such an aluminium foil layer also provides optimum light-protection properties which are required in many cases. For this reason aluminium foil has found wide application as a constituent in packing laminates, especially in connection with the type of packages which are formed by folding, welding together to bags or cushions etc. It is a disadvantage of aluminium foil laminate of the aforementioned type, however, that it cannot be stretched or deep-drawn or at least deep-drawn only to a very small degree. Most plastic materials can be readily subjected to a plastic deformation by heating the plastic material, whereupon it can be formed through drawing, blowing etc., the forming taking place so that the surface of the plastic material is extended with simultaneous thickness reduction of the material. In the formation of a large number of packages a plastic material is formprocessed by stretching, deep-drawing or blowing so as to produce a cavity into which the contents can be introduced. If such a forming were to be carried out on a laminate including an aluminium foil layer, the aluminium foil layer would break almost immediately, as a result of which the gas-tightness characteristics would of course, be lost.
Since aluminium foil possesses many characteristics which are desirable in connection with packaging, attempts have been made to develop an aluminium laminate suitable for deep-drawing. In principle it is known that aluminium in thicker layers readily can be formed by deep-drawing, but the problem is that rolled foils of thicknesses of 5-10 .mu.m break at relatively modest elongation. Successful attempts have been made, however, to manufacture relatively shallow troughs, plates and the like from laminated material comprising an aluminium foil, and a report on these experiments was published in the journal Verpackungsrundschau no. 4 of the year 1977 (N. Buchner, D. Liede, W. Brose--Aluform--Eineneue Packung aus Aluminiumkunststoff vebunden). In this paper an account is given, among other things, of how the constitutional structure of the aluminium material affects the deep-drawing capacity of the material. A curve is also included showing the breaking elongation of aluminium foil as a function of the thickness of the aluminium foil. It is evident from this curve that the breaking elongation for aluminium foil of a thickness of 5-10 .mu.m is as low as 2-3%, while the breaking elongation of aluminium foil with a thickness of 140 .mu.m is as high as 32%, after which the breaking elongation, in principle, does not increase with increasing thickness of the aluminium foil. In the article it is also explained, how the deep-drawing capacity of a laminate containing aluminium foil can be increased by attaching the aluminium foil in the laminate to a polypropylene film, which imparts a very good and uniform transfer of tensile strength to the aluminium foil during further form-processing.
The problem which has been stated has been solved by the present invention which has the characteristics furnished in the description below.